It is a teaching about true yoga, that is to say, Yoga considered as a whole : the 8 branches as developped by Patanjali. In this really authentic view, the core method is meditation, intended to rise inner silence. Then we can add, optionally and if we have time, spinal breathing pranayama. Then again, we can add some asanas. Each time, the new practice is used to enhance the effectiveness of meditation.

In our occidental countries, these asanas are considered as the core practice.
In an authentic view, asanas are just "options" to enhance something much bigger.

Pranayama is extremely important. There have been links to life expectancy and breaths per minute in the animal kingdom, it seems the species that breath the most breaths per minute have the shortest lives, and vice versa. The yogis believed that humans could extend their lives by 20 years by lowering the average breath per minute from 10-15, to 4-5 per minute. When I take long walks, I sometimes find myself holding my breath without being aware that I am. My heart can be beating fast but I can still breath slow, it goes hand and hand with meditation as well.

Breath retention can lead to a rise of kundalini, the body try to compensate the lack of prana taken from the air by changing its way of working, unlocking in a certain way the kundalini.
And on a other hand, deep meditation can generate the stop of breathing.
Maybe because the inner silence (shiva) wants to marry the ecstatic energy (shakti) by favoring it ?